Dialkylidenecyclobutane/bisimide/triene composition

ABSTRACT

A composition comprising a 1,2-dialkylidenecyclobutane such as 1,2-dimethylenecyclobutane, a triene such as myrcene and a polyimide such as a bismaleimide can be thermally cured to a tough copolymer having a high glass transition temperature.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of Ser. No. 733,947, filed Jul. 22, 1991,now U.S. Pat. No. 5,147,953.

BACKGROUND OF THE INVENTION

This invention relates to thermosettable resin compositions. In oneembodiment, the invention relates to enhancement of the properties ofbisimide/triene compositions.

Advanced composites are high-performance materials made up of afiber-reinforced thermoplastic or thermosettable material.Thermosettable materials useful in advanced composites applications mustmeet a set of demanding property requirements. For example, suchmaterials optimally have good high-temperature properties such as high(above 200° C.) cured glass transition temperature and good mechanicalstrength. For ease of processing in preparing prepregs for compositeparts, the uncured material will ideally have a low (below 120° C.)melting temperature and a wide temperature range of processableviscosity (a wide "processing window").

Bismaleimide resins have superior high-temperature properties but arevery brittle and further tend, because of their high softening points,to require solvents in order to be readily processable. In addition,standard cured bismaleimide resins tend to have high (in the 5-7% range)93° C. water absorption. Addition of thermoplastic or cyanate-terminatedoligomers to bismaleimides increases the toughness but produces uncuredmixtures so high in viscosity that fiber impregnation and processing bystandard thermoset techniques are difficult.

It is thus an object of the invention to provide new thermoset resinmaterials. In one aspect, it is an object of the invention to providecomomomers which provide low-melting bismaleimides which cure tohigh-Tg, tough resins.

SUMMARY OF THE INVENTION

According to the invention, a composition is provided comprising a1,2-dialkylidenecyclobutane, a bisimide and a reactive triene which ischaracterized by a conjugated diene moiety capable of Diels-Alderreaction with the bisimide and an isolated double bond. The inventioncopolymers have superior toughness and can be melt-processed forcomposites applications.

DETAILED DESCRIPTION OF THE INVENTION

The invention composition includes a bisimide of an unsaturateddicarboxylic acid. The preferred bisimides are N,N'-bisimides ofunsaturated dicarboxylic acids which can be represented by the formula##STR1## in which Y is a substituted or unsubstituted divalent groupcontaining at least 2 carbon atoms, preferably 2 to 6 carbon atoms, anda carbon-carbon double bond, and Z is a divalent group containing atleast 1 and generally about 1 to 40 carbon atoms. Z can be aliphatic,cycloaliphatic, aromatic or heterocyclic. A preferred class of bisimidescomprises bismaleimides derived from aromatic amines and can berepresented by the formula ##STR2## in which each R₁ is selectedindependently from H, C₁₋₂ alkyl or halide; R₂ is selected from divalenthydrocarbon radicals containing from about 1 to about 10 carbon atoms,--O--, --SO₂ --, --COO--, --CONH--, --CO-- and --S--; and each R₃ isselected independently from H, C₁₋₃ alkyl and halide. The aromatic ringsmay alternatively be heterocyclic.

Examples of such bisimides include

1,2-bismaleimidoethane

1,6-bismaleimidohexane

1,3-bismaleimidobenzene

1,4-bismaleimidobenzene

2,4-bismaleimidotoluene

4,4'-bismaleimidodiphenylmethane

4,4'-bismaleimidodiphenylether

3,3'-bismaleimidodiphenylsulfone

4,4'-bismaleimidodiphenylsulfone

4,4'-bismaleimidodicyclohexylmethane

3,5-bis(4-maleimidophenyl)pyridine

2,6-bismaleimidopyridine

1,3-bis(maleimidomethyl)cyclohexane

1,3-bis(maleimidomethyl)benzene

1,1-bis(4-maleimidophenyl)cyclohexane

1,3-bis(dichloromaleimido)benzene

4,4'-biscitraconimidodiphenylmethane

2,2-bis(4-maleimidophenyl)propane

1-phenyl-1,1-bis(4-maleimidophenyl)ethane

α,α-bis(4-maleimidophenyl)toluene

3,5-bismaleimido-1,2,4-triazole

and various N,N'-bismaleimides disclosed in U.S. Pat. No. 3,562,223,4,211,860 and 4,211,861. Bismaleimides can be prepared by methods knownin the art, as described in U.S. Pat. No. 3,018,290, for example.

The bisimide resin can contain imide oligomers according to the formula##STR3## in which x is a number within the range of about 0 to about 3.Such oligomers may be present as an impurity in difunctional bisimides.

The preferred bisimide resin is 4,4'-bismaleimidodiphenylmethane.Suitable N,N'-unsaturated bismaleimide resins are commercially availablefrom Technochemie GmbH as Compimide® resins, for example.

The invention composition includes a 1,2-dialkylidenecyclobutane,including those which can be described by the structural formula##STR4## in which each R is selected independently from hydrogen, C₁₋₁₀alkyl, halo, aryl, alkoxy, aryloxy, alkylthio, arylthio anddialkylamino. The presently preferred 1,2-dialkylidenecyclobutane,because of the superior properties of a bisimide copolymer preparedtherewith, is 1,2-dimethylenecyclobutane, which is defined by the aboveformula when each R is hydrogen.

In general, dialkylidenecyclobutanes can be prepared by the thermaldimerization of the corresponding allenes in a recirculating hot-tubereactor. Specifically, the process will generally be carried out bycirculating a stream of gaseous allene through a tube reactor at450°-600° C. with a residence time in the hot zone of 0.1 to 10 seconds.Downstream from the hot zone, the stream is cooled sufficiently tocondense the dialkylidenecyclobutane. Unchanged allene (combined with afresh makeup stream) is recirculated back to the hot zone by a pump.Such a process is described for 1,2-dimethylenecyclobutane in Chernykhet al., Neftepererab. Neftekhim., 1981 (7), 48-50. Synthesis of1,2-dimethylenecyclobutane is also illustrated in Example 1 herein. Theallene starting material can be produced by pyrolysis of isobutylene orby isolation from a hydrocarbon mixture such as a refinery crackerstream.

The invention composition includes a reactive triene characterized by aconjugated diene moiety capable of unhindered Diels-Alder reaction withthe Y moiety of the above-described bisimide and an isolated double bondseparated from the conjugated pair by a chemical linking group. Suchtrienes can be represented by one of the formulas I and II: ##STR5## inwhich each R¹ is selected independently from hydrogen and C₁₋₃ alkyl,and R² is a divalent linking group. R² can be, for example, alkylene,preferably C₂₋₁₂ alkylene; (CH₂)_(n) R³ (CH₂)_(n) ; and O--R³ --O--, inwhich n is an integer from 1 to about 6 and R³ is C₁₋₁₂ alkylene,carbonyl, phenylene, and the like. The preferred R² linking group isC₂₋₁₂ alkylene, as in myrcene and trans-1,3,7-octatriene. The conjugateddiene group must be capable of Diels-Alder reaction with the Y moiety ofthe bisimide and must not contain substituents in positions which wouldblock the approach of a Diels-Alder dienophile when the diene group isin the cisoid confirmation, as in cis-1,3,7-octatriene, for example. Theisolated double bond should be adjoined by at least one hydrogen-bearingcarbon atom. Because of the fracture toughness and high-temperatureproperties achievable in the resulting bisimide copolymer, myrcene isthe preferred triene comonomer.

The polyimide, triene and 1,2-dialkylidenecyclobutane monomers may becombined in any manner desired, such as melt, solution or powderblending. The preferred technique, when sufficiently large quantities ofmonomers are used, involves heating a mixture of the solid polyimide andtriene and the liquid 1,2-dialkylidenecyclobutane with stirring at atemperature above the respective melting points but below thepolymerization temperature of any monomer, until the mixture becomes ahomogeneous melt. The melt may optionally be held at temperatures aboveabout 120° C. for desired periods of time in a process ofprepolymerization to increase the crystallization resistance of the meltand/or to increase its viscosity to desired levels. The mixture can thenbe poured directly into a mold for polymerization, or it can be cooledfor later polymerization. For small quantities of monomers, however,excessive amounts of the dialkylidenecyclobutane may volatilize duringthe melt reaction, upsetting the desired stoichiometric balance. Inthese cases, it is preferable for the monomer mixture to be processed ina two-step process in which the monomer mixture is reacted in a solvent,with the solvent then evaporated and the adduct melted and cured tosolid polymer without solvent.

The relative amounts of the monomers will depend upon the cured anduncured properties desired. In general, optimum properties will beachieved with a ratio of moles (dialkylidenecyclobutane +1.5×triene):bisimide within the range of about 0.5:1 to about 2:1, preferably about0.8:1 to about 1.5:1.

The composition may contain an optional free radical inhibitor toinhibit free radical polymerization of the bisimide monomer. Generally,the free radical inhibitor will be present in the composition in anamount within the range of about 0.0002 to about 0.02 moles per mole ofthe bisimide, preferably from about 0.001 to about 0.01 moles. The freeradical inhibitor can be added to the monomers in any manner effectivefor intimate blending of the monomers and free radical inhibitor. Freeradical inhibitors include phenols such as t-butylcatechol, hydroquinoneand p-methoxyphenol; quinones such as 1,4-benzoquinone and1,4-naphthoquinone; polynitro aromatics such as picric acid and2,4,6-trinitrotoluene; hydroxylamines such as diethylhydroxylamine;stable radicals such as di-t-butylnitroxide or diphenylpicrylhydrazyl;and certain polycyclic heterocycles such as phenothiazine. The preferredfree radical inhibitor is phenothiazine.

Polymerization is effected by heating the mixture to a temperatureeffective to initiate opening of the cyclobutene ring (formed by theinitial Diels-Alder reaction of the diene group of thedialkylidenecyclobutane with the dienophilic double bond) to form atransient diene which rapidly reacts with available maleimide groups.This temperature is generally at least about 150° C., preferably about180° to about 350° C., held for a time of about 0.5 hour or more (withthe required cure time dependent on the temperature-staging programused).

In order to achieve optimum properties, a mixture of the monomers andfree radical inhibitor is heated at a temperature near or above theultimate (fully-cured) glass transition temperature of the copolymer fora time sufficient to achieve essentially complete reaction of themonomers. "Essentially complete" reaction of the monomers has beenreached when no further reaction exotherm is observed by differentialscanning calorimetry (DSC) upon heating the copolymer. The time of theheat treatment, or "post-cure," will vary depending upon the monomers,the degree of pressure applied and any pre-curing of the monomermixture. Preferably, this post-cure is at or above the ultimate Tg, butwill always be at a temperature lower than the temperature at whichdegradation of the copolymer will occur at significant rates.

The copolymers are useful in adhesives, coatings and as resin matricesfor composites in aerospace and electronics applications, includinglarge structural parts and circuit boards. Based on their long shelflife and relatively low melting point, some of the uncured mixtures areuseful for making tacky prepregs which can then be molded intocomposites. They are also suitable for low-solvent or solventless liquidresin processing methods such as filament winding, resin transfermolding and pultrusion if the mixtures are heated to providesufficiently low viscosity for fiber impregnation.

Electrical applications for the invention compositions includeencapsulation of electronic devices and electrical lamination forcircuit board manufacture. In encapsulation, the composition willusually be combined, generally by melt-blending, with a suitable inertfiller such as particulate silica. For lamination, the composition willbe applied, in organic solution or in a solventless melt, to a suitablereinforcement such as glass fiber, and partially cured to form anelectrical prepreg, which will subsequently be fabricated into afully-cured laminate.

For preparation of reinforced laminate materials, a fibrous substrate ofglass, carbon, quartz, poly(p-phenyleneterephthalamide), polyester,polytetrafluoroethylene, poly(p-phenylenebenzobisthiazole), boron, paperor like material, in chopped, mat or woven form, is impregnated with abisimide/dialkylidenecyclobutane composition in molten or solution form.A prepreg is formed by heating the impregnated substrate in an oven at atemperature sufficient to remove the solvent and to partially curewithout gelation, or "B-stage," the resin system, generally about 120°C. to about 180° C., preferably about 135° to about 175° C., for a timeof up to about 2 hours, preferably about 10 to about 40 minutes. Alaminate is fabricated by subjecting a set of layered prepregs toconditions effective to cure the resins and to integrate the prepregsinto a laminated structure. The laminate can optionally include one ormore layers of a conductive material such as copper.

Laminating generally involves subjecting the prepregs to a temperatureabove about 175° C., preferably from about 180° to about 350° C., for atime of at least about 10 minutes, at a pressure within the range ofabout 50 to about 500 psi.

For some laminating applications, it may be advantageous to heat treat,or upstage, the monomer mixture prior to application to a laminatingsubstrate, particularly if the mixture will be stored prior to use.Suitable heat treatment involves subjecting the mixture to an elevatedtemperature for a time sufficient to cause sufficient reaction andviscosity increase to inhibit crystallization of either or both monomersfrom the mixture upon storage, but not sufficient to gel thecomposition. Such heat treatment conditions generally include atemperature of at least about 120° C., preferably about 135° to about175° C., for a time of at least about 10 minutes, preferably about 12 toabout 90 minutes. The resulting mixture will be less tacky and lesssusceptible to crystallization of the components with storage.

EXAMPLE 1 Preparation of 1,2-Dimethylenecyclobutane.

A recirculating apparatus for the thermal dimerization of allene wasdesigned as follows. The heated reactor was a bank of approximately 110segments (each about 30 cm long) of stainless steel tubing 1.27 cm inoutside diameter. The segments were arranged vertically in series andconnected to one another by U-shaped stainless steel connectors to whichthey were welded. The volume of the heated portion of the reactor wasabout 3.4 liters. The bank of tubes was immersed in a fluidized bed ofaluminum oxide particles. Thermocouples wedged between the connectors ofthe reactor at various points allowed one to monitor the walltemperature of different segments of the reactor.

Downstream from the reactor was a cold trap containing a cooling fluidat approximately -65° C. above a flask which functioned as a gas-liquidseparator. Downstream from the first trap was a second trap filled withdry ice in dichloromethane, guarding the outlet to the system (throughan oil bubbler) to condense any allene which otherwise could haveescaped from the system. Condensed allene from this second trap fellinto the gas-liquid separator. The condensed material (allene dimers andsome of the allene) from the traps fell to the bottom of the separatorand then flowed through a fluoropolymer tube into a reservoir for liquidallene and allene dimers. Sufficient heat was applied to this reservoirto keep the allene boiling gently. The allene not condensed by the coldtraps was combined with that evaporating from the reservoir. This streamof recovered allene was passed through a filter into a diaphragm pumpwhich recirculated the allene back into the hot tube. A makeup stream offresh allene from a cylinder was also introduced into the loop justupstream from the recirculation pump.

The system was first purged with nitrogen. The power to the fluidizedbed was turned on and its temperature was brought to 450°-470° C. Allenewas introduced into the system from the allene cylinder at a rate of80-100 g/hr. The allene supply from the cylinder was shut off two tothree hours before the end of a dimerization run in order that theallene present in the system could be used up, with little alleneremaining in the reservoir at the end. At the end of the day, the powerto the fluidized bed was turned off, the system was allowed to cool, andthe accumulated dimer was poured into a bottle and weighed.Approximately 3 g of phenothiazine was added per kilogram of dimer toinhibit polymerization of the 1,2-dimethylenecyclobutane. The crudedimer was then analyzed by gas chromatography for peaks corresponding totwo allene dimers, 1,2-dimethylenecyclobutane (1,2-DMCB) and1,3-dimethylenecyclobutane (1,3-DMCB), and a component shown by massspectrometry to have a molecular formula of C₉ H₁₂ (an allene trimer).Data from seven hot tube reaction runs are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                   Crude      GC analysis                                               Reaction                                                                           Allene                                                                            dimer Crude                                                                              1,3-DMCB,                                                                           1,2-DMCB,                                                                           C.sub.9 H.sub.12                        Reaction #                                                                          time, hr.                                                                          used, g                                                                           produced, g                                                                         yield, %                                                                           %     %     peak, %                                 __________________________________________________________________________    1     8.0  558 443   79.4 8.4   67.0  15.0                                    2     15.8 1197                                                                              881   73.6 8.1   75.0  11.0                                    3     11.3 862 753   87.3 8.3   73.4  11.4                                    4     11.2 824 647   78.5 8.3   71.6  14.0                                    5     11.8 932 806   86.5 8.5   68.7  15.4                                    6     11.4 909 746   82.1 8.4   68.0  16.2                                    7     11.0 872 724   83.0 8.5   69.0  15.7                                    __________________________________________________________________________

The products of the seven runs in Table 1 were flash-distilled undervacuum to remove tars and were subsequently distilled under reducedpressure in 2.54 cm Oldershaw columns with 30 plates. The resultingdistilled fractions and similarly-obtained DMCB cuts were used in thefollowing examples.

EXAMPLE 2

Bis(4-maleimidophenyl)methane (COMPIMIDE® MDAB) (2508.6 grams) andphenothiazine (7.54 grams) were mixed and wetted thoroughly with myrcene(SCM Glidco P&F grade) (619.8 grams). The wet powder was added inportions to a 3.5-liter stainless steel beaker with a handle held on ahot plate. The mixture was stirred as additional wetted powder was addeduntil the contents were liquid and homogeneous (this required about 10minutes; maximum liquid temperature reached was 146° C.). The mixturewas then poured out into aluminum trays and allowed to cool to roomtemperature and solidify. The solidified adduct was broken into chunksand used in the following section.

Four mixtures (mixtures 1-4 in Table 2 below) were prepared as follows.To 500-mL glass bottles were added the amounts shown of COMPIMIDE® MDAB(the bismaleimide of 4,4'-methylenedianiline), a distilleddimethylenecyclobutane fraction containing mostly 1,2-isomer, theCOMPIMIDE® MDAB-myrcene adduct prepared above, phenothiazine andMonsanto PC-1344 defoamer (an acrylic oligomer, added to preventexcessive foaming during vacuum degassing and to enable the preparationof void-free castings) along with 180 grams of dichloromethane solvent.In these mixtures, the number of moles of bismaleimide (other than thatpre-incorporated into the myrcene adduct) was equal to the number ofmoles of 1,2-dimethylenecyclobutane. The bottles were placed on rollersand rolled overnight (or longer) at room temperature to allow completionof the first-stage Diels-Alder reaction between the1,2-dimethylenecyclobutane and the maleimide groups of the bismaleimide.The mixtures were poured into 250-mL Erlenmeyer flasks with a vacuumconnection. The flasks were placed in a 125°-150° C. oil bath and thecontents were swirled as solvent, 1,3-dimethylenecyclobutane, and othervolatile unreacted materials were removed, first at atmospheric pressureand then under mechanical pump vacuum for a few minutes until bubblinghad essentially stopped. The degassed molten mixtures were then pouredinto a two-piece rectangular stainless steel mold with a 1/8" (3.175 mm)thick cavity, with the mold parts separated by a gastight siliconerubber gasket such that the mold could be pressurized during cure. A fewgrams of each uncured sample were kept as a retain for characterizationof uncured properties. The mold was then placed into an oven andpressurized with nitrogen to 750 kPa (˜95 psig) and the systems werecured for one hour at 150° C., followed by ramping linearly to 290° C.over a period of 3.5 hours and then holding for one hour at 290° C. Afifth casting (#5) was prepared by curing the bismaleimide-myrceneadduct prepared above without adding any DMCB or additionalbismaleimide. Properties of the castings (and the uncured systems) areshown in Table 2.

One can see from Table 2 that incorporation of the myrcene into theBMI-DMCB composition lowers the temperature at which a desirable fiberimpregnation viscosity of 1 Pas is reached. This temperature, sometimesdefined as the lower limit of the "processing window," is lowered inline with the percentage of myrcene incorporated into the system. Thetemperature at which a viscosity of 1 Pas is reached again with risingtemperature, sometimes defined as the upper limit of the "processingwindow," is similarly raised with increasing myrcene incorporation,widening the processing window further. Tg is also raised somewhat, andMEK and dichloromethane absorption are lowered somewhat, with increasingreplacement of DMCB by myrcene. Low room temperature dry modulus is adisadvantage of some unmodified DMCB-BMI systems. One can see from Table2 that myrcene incorporation similarly raises room temperature drymodulus in line with the extent of replacement of DMCB by myrcene (93°C. wet modulus is also raised slightly). Even though myrcene additionproduced loss of toughness, all the BMI/DMCB/myrcene blends had fracturetoughness values higher, and two considerably higher, than that of thecontrol BMI/myrcene copolymer containing no DMCB (casting #5).

                                      TABLE 2                                     __________________________________________________________________________    Experiment #     1     2     3     4     5                                    __________________________________________________________________________    Composition:                                                                  Bismaleimide, grams                                                                            73.10 61.29 40.86 20.43                                      moles            0.2040                                                                              0.1710                                                                              0.1140                                                                              0.0570                                     1,2-Dimethylenecyclobutane                                                    (DMCB):                                                                       Crude distillate, grams                                                                        19.78 15.62 10.41 5.21                                       % 1,2-isomer in crude                                                                          82.606                                                                              87.723                                                                              87.723                                                                              87.723                                     distillate (GC area)                                                          Net 1,2-isomer, grams                                                                          16.34 13.70 9.13  4.57                                       moles            0.2039                                                                              0.1710                                                                              0.1140                                                                              0.0570                                     Bismaleimide/myrcene adduct, g                                                                       25    50    75    100                                  Phenothiazine, g 0.20  0.20  0.20  0.20  0.20                                 Monsanto PC-1344 defoamer, g                                                                   0.20  0.20  0.20  0.20  0.20                                 Uncured properties:                                                           Temperature, °C., at which                                                              125   124   120   109   101                                  viscosity reaches 1 Pa.s on                                                   heatup                                                                        Temperature, °C., at which                                                              193   206   219   266   282                                  viscosity again exceeds 1                                                     Pa.s                                                                          Cured properties:                                                             Rheometrics tan δ peak, °C.                                                       291   295   325   345   345                                  R.T. dry flexural (ASTM                                                       D-790):                                                                       Yield strength, MPa                                                                            122 ± 1                                                                          123 ± 1                                                                          125 ± 2                                                                          126 ± 9                                                                          124 ± 6                           Tangent modulus, GPa                                                                           2.67 ± 0.02                                                                      2.73 ± 0.03                                                                      2.85 ± 0.05                                                                      2.98 ± 0.04                                                                      3.06 ± 0.08                       Break elongation, %                                                                            >6.5  >6.5  >6.5  >6.0  5.2 ± 0.4                         93° C. wet flexural (ASTM                                              D-790):                                                                       Yield strength, MPa                                                                            81 ± 1                                                                           82 ± 2                                                                           86 ± 11                                                                          93 ± 1                                                                           85 ± 13                           Tangent modulus, GPa                                                                           2.33 ± 0.02                                                                      2.31 ± 0.02                                                                      2.38 ± 0.07                                                                      2.34 ± 0.02                                                                      2.41 ± 0.05                       Break elongation, %                                                                            >6.5  >6.5  >5.0  >6.0  4.1 ± 1.0                         Compact tension fracture                                                                       2.93 ± 0.13                                                                      1.61 ± 0.09                                                                      1.04 ± 0.03                                                                      0.75 ± 0.04                                                                      0.68 ± 0.01                       toughness, K.sub.q,                                                           MPa-m.sup.1/2  (ASTM E 399-83)                                                Dielectric constant, 1 MHz                                                                     3.40  3.22  3.12  3.33  3.33                                 (ASTM D229/15)                                                                Dissipation factor, 1 MHz                                                                      0.0156                                                                              0.0139                                                                              0.0125                                                                              0.0139                                                                              0.0138                               (ASTM D229/15)                                                                93° C. H.sub.2 O pickup, %:                                            1 day            1.66  2.38  2.76  2.25  2.21                                 2 weeks          2.28  3.00  3.35  2.71  2.55                                 Room temp. methyl ethyl                                                       ketone pickup, %:                                                             1 day            0     0     0     0     0                                    2 weeks          1.69  0.55  0     0     0                                    Room temp. CH.sub.2 Cl.sub.2 pickup, %:                                       1 day            169   disint.                                                                             17.1  6.11  3.24                                 2 weeks          179         75.2  61.4  54.5                                 __________________________________________________________________________     .sup.a Only the 1,2isomer contained in the crude 1,2dimethylenecyclobutan     distillate was considered as part of total system solids. Other component     in the distillate were made up primarily of DielsAlder unreactive,            1,3isomer and were not counted as contributing to solids.                

I claim:
 1. A composition comprising(a) a difunctional bisimide of anunsaturated dicarboxylic acid represented by the formula ##STR6## inwhich Y is a substituted or unsubstituted divalent moiety containing atleast 2 carbon atoms and a carbon-carbon double bond, and Z is adivalent linking group; (b) a triene which contains both a conjugateddiene moiety capable of unhindered Diels-Alder reaction with a Y groupof the bisimide and a carbon-carbon double bond separated from theconjugated pair by a divalent linking group; and (c) a1,2-dialkylidenecyclobutane represented by the formula ##STR7## in whicheach R is selected independently from the group consisting of hydrogen,C₁₋₁₀ alkyl, halo, aryl, alkoxy, aryloxy, alkylthio, arylthio anddialkylamino.
 2. The composition of claim 1 in which the triene isrepresented by one of the formulas I or II: ##STR8## in which each R¹ isselected independently from the group consisting of hydrogen and C₁₋₃alkyl and R² is a divalent linking group.
 3. The composition of claim 2in which component (a) is a bismaleimide.
 4. The composition of claim 3in which component (c) is 1,2-dimethylenecyclobutane.
 5. The compositionof claim 4 in which the molar ratio[dimethylenecyclobutane+(1.5×triene)]:bismaleimide is within the rangeof about 0.5:1 to about 2:1.
 6. The composition of claim 1 in whichcomponent (a) comprises 4,4'-bismaleimidodiphenylmethane and component(c) is 1,2-dimethylenecyclobutane.
 7. The composition of claim 3 whichfurther comprises an effective amount of a free radical inhibitor forthe bismaleimide.
 8. The composition of claim 1 which further comprisesa fibrous reinforcing agent.
 9. The composition of claim 1 which furthercomprises particulate silica.
 10. A polymeric composition comprising theproduct of contacting, at a temperature of at least about 180° C.,monomers comprising(a) a bisimide of an unsaturated dicarboxylic acidrepresented by the formula ##STR9## in which Y is a substituted orunsubstituted divalent moiety containing at least 2 carbon atoms and acarbon-carbon double bond, and Z is a divalent linking group; (b) atriene which contains both a conjugated diene moiety capable ofunhindered Diels-Alder reaction with a Y group of the bisimide and acarbon-carbon double bond separated from the conjugated pair by adivalent linking group; and (c) a 1,2-dialkylidenecyclobutanerepresented by the formula ##STR10## in which each R is selectedindependently from the group consisting of hydrogen, C₁₋₁₀ alkyl, halo,aryl, alkoxy, aryloxy, alkylthio, arylthio and dialkylamino.
 11. Thecomposition of claim 10 in which the molar ratio[dialkylidenecyclobutane+(1.5×triene)]:bisimide is within the range ofabout 0.8:1 to about 1.2:1.
 12. The composition of claim 10 in which thetriene is myrcene.
 13. The composition of claim 10 in which component(a) is a bismaleimide and component (c) is 1,2-dimethylenecyclobutane.14. The composition of claim 13 in which the triene is myrcene.
 15. Aprepreg comprising the composition of claim 1 and a fibrous substrate.16. A process for preparing a copolymer comprising the steps of(a)preparing a mixture of a difunctional bisimide of an unsaturateddicarboxylic acid, myrcene and from about 0.6 to about 2.5 moles, permole of the difunctional bisimide, of a 1,2-dialkylidenecyclobutanerepresented by the formula ##STR11## in which each R is selectedindependently from the group consisting of hydrogen, C₁₋₁₀ alkyl, halo,aryl, alkoxy, aryloxy, alkylthio, arylthio and dialkylamino; and (b)heating said mixture to a temperature within the range of about 180° toabout 350° C. for at least about 0.5 hour.
 17. The process of claim 16in which the difunctional bisimide comprises4,4'-bismaleimidophenylmethane.
 18. The process of claim 17 in which thedifunctional bisimide further comprises at least one of2,4-bismaleimidotoluene or 1,3-bismaleimidobenzene.
 19. The process ofclaim 16 in which said mixture further comprises phenothiazine.